Device which follows the position of the sun

ABSTRACT

A solar installation is described. It has a solar element and a diffractively and/or refractively operating optical apparatus by way of which the incident sunlight is passed directedly perpendicularly on to the solar element. The optical apparatus is caused to track the position of the sun by way of a tracking device. 
     The optical apparatus uses a light-deflecting foil having portions of different natures with a structure having a different optical action. By virtue of the action of the tracking device the holographic foil is moved relative to the solar element so that different portions of the foil move into an operative position above the solar element in succession during the tracking procedure.

This application claims priority based on an International Applicationfiled under the Patent Cooperation Treaty, PCT/DE00/01778, filed on May26, 2000, and German Application No. 199 24 783.8, filed on May 29,1999.

BACKGROUND OF THE INVENTION

The invention concerns a diffractively and/or refractively operatingoptical apparatus for passing incident light, preferably sunlight, on toa receiver, preferably on to a solar element. The optical apparatusincludes a tracking device which is controlled based on the variationwith respect to time of the relative position of the light source andthe receiver, preferably based on the position of the sun.

Optical apparatuses that track the direction of sunlight are used insolar installations. They are associated with the solar elements, tomake the most efficient possible use of the sunlight; in order to directthe incident sunlight on to the solar element on an angle as close toperpendicular as possible. In the practical context of solarengineering, this is accomplished by using focusing systems with lensesand parabolic mirrors, which suitably deflect and concentrate the light.In order to achieve an optimum effect, these systems track the movementof the sun. This requires large and expensive tracking devices, whichprecisely control the position of these generally bulky and heavyoptical apparatuses.

A press release in the newspaper “Frankfurter Allgemeine Zeitung,”supplement No. 144 of 28th Jul. 1994, contained a report about the useof holographic foil for applying sunlight to solar cells. Theholographic foil is intended to replace conventional prisms and lenses.The foil divides up the light spectrum and directs the divided up lightto solar cells, which are specifically designed for the respectivespectral range.

DE 31 41 789 A1 discloses a sun ray concentrator having a body which isin the form of a prism and has a material layer with a hologramstructure on the entrance face and on the reflection face. Theparameters of the hologram structure are so selected that the radiationis passed by means of the hologram into the prism where it is redirectedin such a way that it issues focused at a plurality of end faces of theprism. In that situation, the radiation is concentrated and at the sametime the arrangement provides for division into the various spectralranges, with concentration of the various spectral ranges on the variousray exit faces. This allows light to be directed to specificphotoconverters for the respective spectral range. However, the sun rayconcentrator consisting of prisms suffers from the above-describeddisadvantages in terms of tracking. In addition, because of the prisms,shadow effects occur which reduce the conversion rate.

U.S. Pat. No. 4,054,356 A1 discloses a sun ray concentrator which is inthe form of a hologram of a light spot source. The focal point of thehologram lens, however, is found to be so large that, for the purposesof arranging a receiver for concentrated radiation at the focal point ofthe lens, an auxiliary device connecting the receiver to the lens isrequired. In addition, this arrangement involves irregular distributionof energy at the surface of the receiver.

DE 30 12 500 A1 discloses a retroreflector for use in light barriers andlight curtains. The reflector uses diffraction gratings which are formedby holographic procedures in a photosensitive material. When thereflector is illuminated, the radiation impinging thereon is reflectedand focused outside the reflector plate with the hologram.

SUMMARY OF THE INVENTION

The object of the invention is to provide an optical apparatus of thekind set forth in the opening part of this specification, which is of asimple structure and affords the respectively desired light deflectionand/or light concentration effect. The optical apparatus providesparticularly efficient conversion of light for uses in solarinstallations.

The optical apparatus of the invention includes a transparent orreflective optical body having diffractive and/or refractive and/orholographic regions. In a first embodiment, “combination (a),” theoptical body has portions of a different nature with respect to one ormore of its optical parameters along the tracking direction. Thedifferent portions of the optical body can be brought into and out ofthe operative position by the action of the tracking device on theoptical body and/or the receiver. The tracking device controls therelative position of the optical body with respect to the receiver. In asecond embodiment, “combination (b),” the optical body is in the form ofa foil and/or is on a foil, which can be tracked by way of the trackingdevice. The tracking device controls the with relative position of theoptical body with respect to the receiver by rolling up and unrollingthe foil.

In accordance with combination (a), the optical body includes differentregions selected from diffractive regions, refractive and holographicregions, which have portions of a different nature in terms of theiroptical parameters. This allows precise tracking to be implemented in aparticularly simple manner. The tracking device which acts on theoptical body and/or on the receiver produces a relative movement betweenthe optical body and the receiver. The tracking movement causes thedifferent portions of the optical body to move successively into theoperative position. The portion of the optical body, which is in theoperative position, directs the light incident at that time on to thereceiver at the desired irradiation angle or with the desiredconcentration.

In combination (b), the optical body is in the form of a foil and/or ison a foil, which can be rolled up and unrolled by way of the trackingdevice. This design provides fundamental advantages in terms ofsimplicity of structure and costs.

Particular advantages are enjoyed when the invention is used in solarinstallations. The receiver is in the form of a solar element which canremain stationary while the position of optical device is controlled totrack the position of the sun. Corresponding advantages are enjoyed whenthe invention is used in hothouses.

The body of the optical apparatus, which includes different regionsselected from diffractive regions, refractive regions and holographicregions, preferably has a flat light entrance face and a flat light exitface. The sunlight impinges on the light entrance face at a given angleof incidence depending on the instantaneous position of the light sourcerelative to the receiver. In solar installations, the angle of incidencedepends on the position of the sun. The incident light passes throughthe body and is deflected or concentrated so that the light issues fromthe body at the light exit face and is passed to the receiver at a givenexit angle or with a given concentration. The optical parameters of thebody are selected to provide a desired exit angle or concentration. Whenthe invention is used in solar engineering applications, the opticalparameters of the body provide an exit angle that makes optimum use ofthe sunlight by directing the sunlight on to the solar element at themost efficient irradiation angle. Preferably, an irradiation angle of90°, or maximum concentration, is achieved.

The portions of the optical body, which differ in terms of their opticalparameters, can be arranged on or in the body in mutually juxtaposedrelationship in the tracking direction. The portions can be in the formof portions which blend continuously into each other or in the form ofseparate discrete portions. An arrangement with a continuous transitionof the portions affords advantages in applications that use continuoustracking. Particular advantages in that respect are achieved if thevariation in the optical parameters in the tracking direction is alsocontinuous with a steady progression.

In preferred embodiments, the optical body or the foil has at least oneregion in layer form, with a light diverting and/or concentratingstructure. The optical body can be provided with holographic elements,for example the body may have a preferably layered region having ahologram structure. The portions which differ in terms of the opticalparameters may be implemented by the portions having different hologramstructures. Instead of or in addition to the hologram structure, theoptical body may have a structure of a diffractive lens or a diffractivemirror, which is used to substantially concentrate the light. In orderto minimize reflection losses at the optical body or the foil, theoptical body or the foil can be de-reflected on the side which facestowards the light source.

The body can be in the form of a rigid or flexible body. Particularadvantages are attained when using a holographic foil. The foil can alsobe in the form of a concentrator foil with the structure of adiffractive lens or a diffractive mirror. The foil may have a pluralityof regions involving different lens structures or different mirrorstructures, those regions being arranged in succession in the trackingdirection.

Tracking can be implemented in a particularly simple manner byassociating the regions of the foil which are different along thetracking direction with at least one solar element. In this embodimentof the invention, a first one of the regions co-operates for a firstperiod of time of one or more days with a solar element and a secondregion of the foil which is adjacent to the first region co-operates fora subsequent second period of time of one or more days with the solarelement. For that purpose, the foil may have regions which can beassociated with the individual days of a year or half year, preferably365 or 182 or 183 different regions.

In the case of larger solar installations having a large number of solarelements, a particularly simple structure is afforded if it is providedthat a plurality of solar elements are arranged in longitudinal andtransverse rows in a grid arrangement and/or the optical body has aplurality of separate regions which are arranged in longitudinal andtransverse rows in a grid arrangement, preferably in a correspondinggrid arrangement to the solar elements. To implement tracking forcompensating for the variation in the position of the light source, thegrid arrangement of the solar elements and/or the regions of the opticalbody is turned through an acute angle relative to the tracking directionand/or the direction of movement of the optical body. Tracking withcompensation for the variation in the position of the sun over thecourse of the year can be achieved if an angle of 0.25° is adopted.

If spectral division of the light occurs at the foil, preferably whenthe sunlight passes through the holographic foil, spectrum-specificsolar cells can be used. A plurality of such spectrum-specific solarcells are arranged in mutually juxtaposed relationship so that theindividual light spectra is fed to the respective solar cells.

When using a flexible foil, the tracking device can be configured as aparticularly simple structure that operates reliably and precisely. Thetracking device can be in the form of a foil transport device having atleast one foil storage device which receives and/or delivers the foil,preferably a drum. Preferably, there is a first drum which winds up thefoil during tracking and a second drum which unwinds the foil duringtracking. In this embodiment, a foil portion is arranged, preferably ina tensioned condition, between the first and second drums and includesthe operative portion of the light guide and/or light concentratordevice. To implement tracking, the first drum is driven in rotation byway of a motor drive. The second drum runs synchronously therewith.

In particular arrangements of the tracking device, there is provided afirst transport device which moves the optical body along its mainextent. In addition, there can be provided a second transport devicewhich moves the optical body at an angle, preferably at a right angle,with respect to its main extent, or which moves it rotatably about anaxis parallel to its main extent. The first or the second transportdevice is controlled in dependence on the time of day, that is to say independence on the position of the sun at the time of day. The othertransport device is controlled in dependence on the time of year, thatis to say in dependence on the position of the sun at the time of theyear.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, features and advantages will be apparent from thedescription hereinafter of a number of embodiments diagrammaticallyillustrated in the drawing in which:

FIG. 1 is a diagrammatic view of a solar installation,

FIG. 2 is a simplified stylized representation of the solar installationof FIG. 1 with the position of the sun in the morning,

FIG. 3 is a simplified stylized representation of the solar installationof FIG. 1 with the position of the sun at midday,

FIG. 4 is a simplified stylized representation of the solar installationof FIG. 1 with the position of the sun in the afternoon,

FIG. 5 is a simplified stylized representation of a solar installationwith a foil with a diffractive lens, in the form of a concentrator, withthe position of the sun at midday,

FIG. 6 is a simplified stylized representation of the solar installationof FIG. 5 with the position of the sun in the afternoon,

FIG. 7 is a simplified stylized representation of a solar installationwith a plurality of solar elements,

FIG. 8 shows a foil with lenses in a grid arrangement,

FIG. 9 is a simplified stylized representation of a solar installationusing the foil of FIG. 8, and

FIG. 10 is a simplified stylized representation of a solar installationwith a foil with a diffractive concave mirror, in the form of aconcentrator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The solar installation in FIG. 1 has a solar element 1. The solarelement 1 can be an individual solar element or it can be a battery ofsolar elements arranged in mutually juxtaposed relationship. The solarelement 1 can be in the form of a photovoltaic solar cell or aheat-generating solar collector. The sunlight 2 which is irradiated onto the solar element 1 is converted by the solar element 1 intoelectrical or heat energy. The energy produced is fed at the output 1 aof the solar element into a network (not shown) or an energy storagearrangement.

Associated with the solar element 1 is an optical apparatus 3 whichpasses the sunlight 2, which is incident at the angle α in dependence onthe position of the sun, on to the surface of the solar element 1. Theoptical apparatus 3 directs the sunlight 2 on to the surface of thesolar element 1 at an angle as perpendicular to the surface of the solarelement 1 as possible in each case, in order to make the most efficientpossible use of the sunlight.

The optical apparatus 3 has a diffractively and/or refractivelyoperating optical body 4 through which the sunlight 2 passes and whichdeflects the sunlight 2. In the illustrated embodiment, the optical body4 is in the form of a transparent holographic foil which is tensioned ata distance above the surface of the solar element 1.

The hologram structure of the irradiated portion of the foil 4, which isarranged above the solar element, is such that the sunlight 2 which isincident on the surface of the foil 4 at the angle α, is deflected onpassing through the foil and issues at an angle β at the underside ofthe foil. The arrangement of the solar element 1 is so selected that thesunlight issuing at the angle β is directed at an angle of preferably90° on to the surface A of the solar element 1. In the illustratedembodiment, the exit angle equals 90° (β=90°) and the foil is tensionedin a plane parallel to the surface A of the solar element 1.

In order to provide efficient use of the sunlight 2 at any position ofthe sun, the optical apparatus 3 has a tracking device 5 which changesthe position of the foil 4 to track the position of the sun relative tothe stationary solar element 1. The tracking device 5 has twosynchronously driven drums 51, 52. The first and second drums 51, 52 arearranged parallel to each other at a mutual spacing. They are eachrotatably supported in stationary mounting pedestals 51 g, 52 g. Thefoil 4 is tensioned between the drums 51, 52, with the two opposite endsof the foil 4 being wound on the drums 51, 52. The drums 51, 52 aredriven in a controlled manner by motor means in such a way that theyrotate synchronously about their drum axis 51 a, 52 a. The direction ofrotation in FIG. 1 is in the clockwise direction so that the foil 4which is tensioned between the drums 51, 52 is transported from left toright in the direction C. The speed of the transport movement iscontrolled based on the variation of the position of the sun during theday.

During the tracking procedure, the foil 4 moves continuously in thedirection C. The foil is wound on to the second drum 52 and unwound fromthe first drum 51. Only the foil portion which is disposed in thetensioned portion above the solar element 1 at any particular time hasthe incident sunlight passing therethrough and only that portion isoperative at the time.

Along its main extent, that is to say in the direction of its surfaceand thus in the tracking direction C, the foil 4 has a varying hologramstructure. The variation in the parameters of the hologram structure isselected so that, with a given predetermined speed of transport ortracking movement, continuous adaptation of the light deflection effectto the angle of incidence α (which is dependent on the position of thesun and the foil 4), is attained. The adaptation of the hologramstructure is such that the exit angle β is approximately constant in thecourse of the day with the angle of incidence α which is dependent onthe position of the sun. That means that the angles β and γ (the angleof incidence for the light at the surface of the solar element 1) areapproximately constant in the course of the day at any position of thesun. Thus, the sunlight is efficiently used at any position of the sun.

In order to provide for adaptation in terms of the time of year,tracking is additionally provided for the angular position of the planeof the foil 4 with respect to the surface A of the solar element 1. Inthis embodiment, the plane of the foil is pivoted, preferably togetherwith the drums 51, 52, about a pivot axis that extends parallel to thesurface of the solar element in the direction C. The drums 51, 52 areprovided with suitable angular tracking by a pivoting mechanism (notshown) which, for example, is arranged in the region of the mountingpedestals 51 g, 52 g.

The rotary drive for the drums 51, 52 for the above-described trackingof the foil in the direction C, with respect to the time of day, isprovided by separate drive motors 51 m, 52 m. The drive motor 51 mdrives the drum shaft 51 a. For that purpose, the drive output shaft(not shown) of the drive motor 51 m is coupled to the drum shaft 51 a byway of a transmission (not shown). The drive motor 52 m drives the drumshaft 52 a in a corresponding manner. The two motors 51 m, 52 m arecontrolled synchronously. The control system controls the transportspeed, that is the tracking of the foil 4 in the direction C, based onthe variation in the position of the sun, with respect to the time ofday.

The foil is retracted at night. That is effected by the drive motorsrunning back in the opposite direction and the foil being unwound fromthe second drum 52 and wound on to the first drum 51.

The pivotal movement of the drums 51, 52, which is required for trackingin terms of the time of year, can also be effected by motor means, byway of a drive motor (not shown) which actuates the above-discussedpivoting mechanism in a suitably controlled fashion.

In the embodiment described above, it was assumed that there was asubstantially continuously varying, light-deflecting hologram structureon the foil or the transparent optical body and, accordingly, that thefoil performed a continuous movement over the solar element. It will beappreciated that it is also possible for the optical body to be providedin a quasi discontinuous manner with a corresponding, light-deflectingstructure, for example in the form of stripes of the same structure. Inthis embodiment, the optical body would have to be moved in acorrespondingly discontinuous or step-wise manner with respect to thesolar element.

The embodiment shown in FIGS. 5 and 6 also involves a solar installationwith a diffractive foil which is guided over a solar element 1. Atracking device 5 controls the position of the diffractive foil so thatit tracks the position of the sun with respect to the time of day bywinding the diffractive foil on and off the drums 51, 52. The drums 51,52 are only diagrammatically indicated and they are substantially largerthan shown in the figures and are mounted at a suitable spacing fromeach other. Unlike the preceding embodiments, the foil 4 used in FIGS. 5and 6 is a foil which concentrates the incident sunlight. This involvesa foil concentrator in the form of a diffractive lens 4 a. On passingthrough the lens 4 a, the incident sunlight is concentrated so that theimage of the sun appears in the solar element 1 arranged at the focalpoint. When using a foil with a lens diameter of between 1 and 5 cm, thespacing of the foil 4 relative to the surface A of the solar element isbetween 10 and 20 cm.

During the day, the foil 4, which is tensioned above the solar element1, is displaced by the tracking device 5 from left to right in theFigures, that is in the East-West direction. This displacement of thefoil 4 allows the image of the sun, which shines down at differentangles according to the time of day, to continuously fall on thestationary solar element 1. FIG. 5 shows the position of the foil 4 withthe angle of incidence of the sunlight 2 approximately perpendicular tothe surface of the foil 4, which would be the relationship at the middaytime. FIG. 6 shows the position of the foil 4 with the sunlight 2 at aninclined angle of incidence, which would be the relationship in theafternoon. As can be seen from FIG. 6, when the direction of sunlight 2forms an inclined angle of incidence, the position of the foil 4 or thelens 4 a is caused to track by displacement towards the right.

The transport speed of the foil 4 for tracking with respect to the timeof day is f× 0.25 per hour, wherein ‘f’ is the focal length of the lens.This tracking speed takes into consideration the change in the angle oflight incidence, which occurs by virtue of the variation in the positionof the sun in terms of the time of day and which is about 15° per hourand results in precise tracking with respect to the time of day.

In modified embodiments, a plurality of solar elements 1 a, 1 b arearranged in succession in the direction of movement of the foil 4. FIG.7 shows such an arrangement of two solar elements 1 a, 1 b. The foil 4,which is tensioned above the solar elements 1 a, 1 b, has two lenses 4a, 4 b which are arranged in succession in the direction of movement Cof the foil. As can be seen from FIG. 7, the lens 4 a is associated withthe solar element 1 a and the lens 4 b is associated with the solarelement 1 b; the lens 4 a illuminates the solar element 1 a and the lens4 b illuminates the solar element 1 b. The spacing S of the solarelements 1 a, 1 b is equal to the spacing between the center lines ofthe lenses 4 a, 4 b. Due to the tracking movement of the foil 4according to the time of day, the image of the sun tracks the positionof the sun at the respective time of day. As a consequence, the image ofthe sun is continuously incident through the lens 4 a to a constantposition on the solar element 1 a and through the lens 4 b to arespective constant position on the solar element 1 b.

In the embodiments of FIGS. 5, 6 and 7, a plurality of solar elements 1a, 1 b and so forth may be arranged in mutually juxtaposed relationshipin one or more rows transversely with respect to the direction ofmovement of the foil. When using foils with annular lenses, a pluralityof lenses 4 a, 4 b and so forth are arranged on the foil in thetransverse direction. The lenses and the associated solar elements of atransverse row are respectively arranged in such a way that the spacingbetween the center lines of adjacent lenses is equal to the spacing ofthe associated adjacent solar elements. In that way, a respective lensof a transverse row is associated with each solar element of atransverse row. The tracking movement of the foil 4, with respect to thetime of day, ensures that each solar element is in each case illuminatedpermanently during the day by way of the lens associated therewith.

FIG. 8 shows a foil portion with lenses 4 a, 4 b, 4 c arranged in araster or grid arrangement on the foil. The lenses are arranged inmutually juxtaposed relationship in longitudinal and transverse rowswhich extend at a right angle to each other. In this embodiment, thegrid arrangement is turned through an angle of about 0.25° with respectto the direction C in which the foil 4 moves and extends. The angle of0.25° corresponds to the daily change in the angle of the sun withrespect to the solar panel; that change in angle is 47°/182 per day. Inthat way, it is possible to compensate for the daily change in angle ofthe sun merely by displacement of the foil in the direction C, that isto say without additional adjustment.

FIG. 9 shows the use of the foil 4 shown in FIG. 8 in a solarinstallation. The foil is tensioned above the solar elements 1 a to 1 farranged in a grid arrangement and is wound on and unwound in thedirection C, that is in the East-West direction. In this case, trackingwith respect to the time of day takes place as in the precedingembodiments by displacement of the foil in the course of the day fromleft to right, as shown in FIG. 9. Throughout the entire day, there isalways a respective lens associated with a given solar element so thatthe solar element is illuminated through that respective lens. Fortracking purposes with respect to the time of year, the foil isdisplaced by a line spacing each day so that each solar element isilluminated by a lens for only one day. On the following day, theadjacent solar element is illuminated through the following lens. Thetracking effect is positively produced upon movement of the foil in thedirection C, by virtue of the grid arrangement being turned through theangle of 0.25°. For, due to the grid arrangement being turned in thatway, with the daily change in the height of the sun with respect to thetime of year above the horizon, relative displacement of the lensesperpendicularly to the direction of propagation is achieved. Thus, thechange in the position of the sun, in terms of the time of year, iscompensated.

This means that, in the embodiment shown in FIG. 9, the tracking actionwith respect to the time of day and also with respect to the time ofyear is effected by the tracking movement of the foil 4 in the directionC. The foil 4 can have 182 different lenses arranged in succession inthe direction of movement and within a year is moved once completely toand fro by way of the tracking device 5, that is to the right in thefirst half-year in FIG. 9 and to the left in the second half-year.

In modified embodiments which, unlike FIGS. 8 and 9, do not have a gridarrangement which is turned through an angle, tracking with respect tothe time of year can also be effected. This is accomplished by pivotalmovement of the plane of the foil about the axis of movement of thesliding motion, or by displacement of the foil in a plane which isinclined with respect to the horizontal and which is towards the sun.When the solar element is arranged on the inclined roof structure of ahouse, which faces towards the sun, tracking with respect to the time ofyear occurs by displacement of the foil parallel to the inclined roofstructure, either upwardly or downwardly.

In the modified embodiment shown in FIG. 10, the foil 4 has adiffractive, concave mirror 4 s in place of the diffractive lens. Thesolar element 1 is arranged on the side of the foil 4, which is towardsthe sun, at a spacing f (where f=focal length) in relation to the foil4. The sunlight which is incident on the mirror 4 s is concentrated sothat the image of the sun falls on the surface A of the solar element.In a corresponding manner to the preceding embodiments, tracking of thefoil is effected by way of a tracking device 5, by displacement of thefoil in the direction C. The mirror foil may also have a plurality ofmirrors 4 s arranged in longitudinal and transverse rows. The foil canbe of a corresponding structure to the foils with a lens structure,which have been described with reference to the embodiments of FIGS. 5to 9. Embodiments similar to FIGS. 4 to 9 are possible with the mirrorfoils.

The heightwise profile of the diffractive lenses and mirrors used in thedescribed embodiments comprises concentric zones of spherical andparaboloidal cross-sections. Instead of or in addition to thoseconcentric structures, the foils 4 may also have transverse structures.The foils may also be operative to deflect and concentrate light at thesame time.

1. An optical apparatus comprising: a receiver comprising a solarelement, wherein incident light from a light source, preferablysunlight, is passed on to the solar element, a transparent or reflectiveoptical body, and a tracking device for moving the optical body in atracking direction, wherein the tracking device is controlled independence on the variation with respect to time of the relativeposition of the light source and the receiver, preferably in dependenceon the position of the sun, wherein the optical body comprises differentregions selected from diffractive regions, refractive regions andholographic regions, which deflect, concentrate or deflect and as wellconcentrate the light, wherein the optical body is formed as a foil, orthe optical body is attached to a further foil, or the optical body isformed as the foil, which is attached to the further foil, wherein theoptical body has a plurality of different portions along the trackingdirection due to the different regions, wherein the different portionsdiffer by one or more optical parameters, wherein the optical body ispositioned by the tracking device with relative movement with respect tothe receiver by rolling up and unrolling the foil, wherein one or moredifferent portions of the optical body are brought into or out of anoperative position, wherein in the operative position the light ispassed via the respective one or more different portions of the solarelement, and wherein the foil is positioned so that light is passed onto the solar element and that at least one of the different portions isassociated with the solar element and a first different portionco-operates for a first period of time of one or more days with thesolar element and a second different portion adjacent the firstdifferent portion co-operates for a subsequent second period of time ofone or more days with the solar element.
 2. The apparatus according toclaim 1, wherein different portions are associated with the individualdays of a year or half-year, preferably 365 or 182 or 183 differentportions.
 3. The apparatus according to claim 1, wherein a plurality ofsolar elements are arranged in longitudinal and transverse rows in agrid arrangement and the optical body has a plurality of separateregions which are arranged in longitudinal and transverse rows in a gridarrangement, preferably in a corresponding grid arrangement to theplurality of solar elements.
 4. The apparatus according to claim 3,wherein the grid arrangement of the solar elements and the separateregions of the optical body is turned through an acute angle relative tothe tracking direction and the direction of movement of the opticalbody, preferably through an angle of 0.25°, to compensate for thevariation in the position of the sun over the year.
 5. The apparatusaccording to claim 1, wherein the tracking device has a first transportdevice which moves the optical body in a first tracking direction alongits main extent.
 6. The apparatus according to claim 5, wherein thetracking device has a second transport device which moves the opticalbody in a second tracking direction in angular relationship with itsmain extent.
 7. The apparatus according to claim 6, wherein the firstand second transport devices are controlled in dependence on the time ofday.
 8. The apparatus according to claim 6, wherein the first or thesecond transport device is controlled in dependence on the time of year.9. The apparatus according to claim 5, wherein the optical body is inthe form of a flexible foil and the transport device is in the form of afoil transport device having at least one foil storage device forreceiving or delivering the foil.
 10. The apparatus according to claim9, wherein there is provided a first drum which winds up the flexiblefoil during the tracking operation and a second drum which unwinds thefoil during the tracking operation and wherein a portion of the foil isarranged in the operative position over the solar element between thefirst and second drums.
 11. The apparatus according to claim 1, whereinthe different portions are arranged in mutually juxtaposed relationshipin the tracking direction, wherein the portions blend continuously intoeach other or are separate discrete portions.
 12. The apparatusaccording to claim 1, wherein the optical body is rigid or flexible. 13.The apparatus according to claim 1, wherein the optical body or the foilhas at least one layered different region with a structure whichdeflects, concentrates or deflects and as well concentrates the light.14. The apparatus according to claim 13, wherein the light-concentratingstructure is provided by a diffractive lens or a diffractive mirror. 15.The apparatus according to claim 14, wherein the foil has a plurality ofdifferent lens structure regions or mirror structure regions which arearranged in succession in the tracking direction.
 16. The apparatusaccording to claim 1, wherein the foil is de-reflected on the sidetowards the light source.